Lay-in ceiling tile or panels formed from various aggregates such as clay and mineral wool have for a long time been limited to a size which will not sag during a long period of use. The conventional limit to this size has been about two by four feet. Sizes larger than this have until recently resulted in noticeable sag, regardless of the particular aggregate which has been used in making the panels. The support of the tile has conventionally been at the edges only, as shown in U.S. Pat. No. 751, a representative ceiling construction.
A recent attempt to overcome the problem involved the use of a spine attached on the site to the back of each ceiling panel, by driving barbs into the back surface at an angle to hold the spine in place. An example of such a construction is disclosed in U.S. Pat. No. 3,557,513. Disadvantages of such a spine stem from the manner in which it has been attached. Because of its considerable height, such a spine must be attached only at the site if the panel is to be readily stacked for shipping. The driving of barbs on the site into an aggregate ceiling panel is at best a time-consuming job which, if not done properly, can result in board breakage. At worst, hard aggregate materials such as expanded perlite or vermiculite will not readily permit the barbs to be driven in.